The subject invention relates to the manufacture of articles having sidewalls of oriented thermoplastic polymeric materials. More particularly, the invention relates to plastic bottles having in their sidewall a high level of circumferential orientation as measured by circumferential orientation release stress (ORS), as hereinafter defined, and to the manufacture of such bottles from closed-end tubes or parisons by blow-molding.
Molecular orientation of thermoplastic polymeric materials is not new. Molecularly oriented film and sheet are widely used and have improved physical properties, including superior impact resistance, increased resistance to creep, increased stiffness, increased resistance to stress rupture and reduced stress crazing, when compared to their unoriented counterparts. Examples of such materials are given in U.S. Pat. No. 3,141,912.
For a given polymer and end use application, there is an optimum level of orientation, which may be below the maximum possible orientation level. For example, impact strength may reach a maximum value as the amount of orientation is increased, with additional orientation resulting in a decreased impact strength. Another example of a property which may deteriorate with attempts to achieve high levels of orientation is optical transparency; certain polymers "stress whiten," giving them a milky appearance.
The amount of orientation in an article formed from a polymeric material is affected by the conditions under which the material is oriented. For example, in a tubular article higher levels of circumferential orientation result from increasing the amount of stretch in either the circumferential or axial direction, by increasing the stretching rate, and by decreasing the stretching temperature.
It has been proposed to form plastic bottles by blow-molding a parison, or closed-end tube. While such proposals have met with some success, it has not generally been economically practicable to form bottles for carbonated beverages by this technique. The reason has been that if the bottle is oriented, by stretching, sufficiently to develop the creep resistance required of containers for carbonated beverages (assuming a wall thickness thin enough to be economic), stress whitening has been observed to occur, making the container unsalable. Impact strength is also found to be undesirably low.
Further analysis of this phenomenon has brought the realization that stress whitening and reduced impact strength, which develop primarily at the inner portion of the bottle wall, is due to the fact that the inside of the parison is stretched to a much higher extent, proportionally, than the outside. It has been found that the degree of orientation is not constant across the bottle wall thickness, but on the contrary varies substantially across the wall, and at or near the inner portion of the wall is sufficiently high to give rise to the stress whitening and low impact strength.